The present invention relates to an optical fiber amplifier having variable gain, to be used in particular in a WDM network, and also to a WDM network and a method of amplifying WDM light signals.
Optical fiber amplifiers have great advantages in optical wavelength multiplexed transmission systems (WDM systems), as they are capable of simultaneously amplifying a number of WDM channels.
Normally such amplifiers are operated in a saturated condition, this implying that they have an approximately constant output power that is independent of the input power. This further implies that the gain of the amplifier is inversely proportional to the input power. This is an advantage in most transmission systems, since amplifiers placed along a transmission path will then automatically adjust their gain to be equal to the losses between the amplifiers.
For WDM systems there is one problem related to this type of amplifier operation. The spectral dependence of the amplifier depends on the population inversion of the amplifying medium. An increasing inversion will shift the amplification towards shorter wavelengths and a gain tilt results. The gain in the saturated amplifier also depends on the degree of population inversion. This implies that the relative gain between the different WDM channels depends directly on the amplification of the amplifier. Gain variations will directly cause an imbalance of the gain between the channels and this will lead to different signal to noise ratios, SNR, at the receiver. The channel with the lowest SNR will set the limit for the performance of the whole system.
Normally an amplifier is dimensioned for one certain application, i.e. the input power level and the gain. When the system is installed and taken into operation the power levels have to be adjusted using attenuators. The remaining gain imbalance has to be handled by the, system within the allowed performance margins. The whole system has to be dimensioned for the worst case where attenuations are maximal in the transmission paths between each amplifier although most of the attenuations might be significantly lower in the actual installation. The total capacity of the transmission system will thus be significantly lower than it potentially could have been.
Methods of providing amplification with equal output power for the different WDM channels have also been presented. The optical WDM spectrum could be demultiplexed, the individual channel power levelled by a set of saturated amplifiers and finally multiplexed again, see U.S. Pat. Nos. 5,452,116 and 5,392,154. The active fiber could be cooled to cryogenic temperatures which will lead to the effect that the gain at each wavelength is saturated individually, so called Spectral Hole Burning, see U.S. Pat. No. 5,345,332. A wavelength tuneable filter with a suitable characteristic could partly compensate for the changes, see the article by R. A. Betts et al., xe2x80x9cSplit-beam Fourier filter and its application in a gain-flattened EDFAxe2x80x9d, Opt. Fiber Communications Conf., TuP4, San Diego, 1995. In specially prepared active fibers the spectral changes have been shown to have a reduced gain dependence, see J. Nilsson, Y. W. Lee and W. H. Choe, xe2x80x9cErbium doped fibre amplifier with dynamic gain flatness for WDMxe2x80x9d, Electron. Lett., Vol. 31, pp. 1578-1579, 1995.
These prior methods seem to be costly or complicated or to have a low performance.
It is an object of the invention to provide an optical amplifier having a variable total gain and a constant spectral dependence within an optical transmission band.
It is another object of the invention to provide an optical amplifier having a constant spectral dependence for a varying power of the input light.
It is another object of the invention to provide a method of amplifying incoming, wavelength multiplexed light signals so that the signals are amplified to produce amplified light signals, where the gain for the different signals has a constant spectral dependence for a varying power of the total incoming light and for a variable total gain.
The problem solved by the invention is thus how to construct an optical amplifier that has a good performance, e.g. has better signal-to-noise ratios for higher input signals than for low signals, and that can be built at a reasonable cost.
Thus, an optical amplifier is provided that has a constant spectral dependence within the optical transmission band of a WDM system while the total gain of the amplifier can be varied without impairing the noise figure or the output power of the amplifier. The remaining spectral gain differences can be equalised with a fixed spectral filter if desired for example for a WDM system. The operation of the amplifier is based on the observation that a gain tilt in one amplifier stage in a two stage optical fiber amplifier can be corrected by a corresponding gain tilt having an opposite sign in another amplifier stage. The amplifier can thus be designed as a two stage amplifier where the first amplifier stage operates as a preamplifier stage in a more or less saturated state. The second stage of the amplifier operates as a power or booster amplifier stage in a fully saturated state with an almost constant output power. The gain of the second stage is adjusted by adjusting the power input to this stage in order to produce a gain tilt having a magnitude equal to and an opposite sign compared to the gain tilt of the preamplifier stage.
The adjustment of the input power of the second stage is preferably achieved by arranging a controllable optical attenuator placed between the two amplifier stages. This attenuator can be either manually or automatically controlled. An automatically controlled attenuator can be mechanically or directly electrically controlled by one of various known techniques.
The gain of the complete amplifier for a fixed input power can further be varied by varying the pump power to the power amplification stage and thereby varying the output power, the attenuation of the attenuator simultaneously then being varied correspondingly to maintain a constant gain of the output amplification stage.
The control of the attenuator can be done in one of various ways. For a manual attenuator the loss is adjusted at installation to be optimised for the actual loss of the line section preceding the amplifier. For an automatically controllable attenuator the loss can either be calculated from the measured input power using the well predictable behaviour of the amplifier, as e.g. presented in the paper by D. Bonnedal, xe2x80x9cEDFA Gain, Described with a Black Box Model,xe2x80x9d in Optical Amplifiers and their Applications, 1996 Technical Digest (Optical Society of America, Washington D.C., 1996), pp. 215-218, or the channel power of two or more channels can actually be measured at the output and the difference there-between can be minimised by a controller algorithm. The channel power measurements can be made using one of several well known techniques such as imposing pilot tones of different frequencies on some or all channels and measuring the relative channel power by frequency discrimination in the electrical domain, see the published European patent application EP-A1 0 637 148 which corresponds to U.S. Pat. No.5,463,487. Alternatively an analysis of the whole or part of the optical spectrum can be made by fixed or scanning optical filters, gratings or interferometers.
By placing an attenuator between the amplification stages the optimal performance of the two stages is not impaired. The lowest possible noise from the preamplifier stage is added to the signal in each case of operation and the full output power is always available from the power amplification stage. This means that if the loss between a pair of amplifiers is lowered the SNR is correspondingly improved. The full potential of the total optical transmission system can be utilised.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.